CHARACTERISTICS OF LIFE
Humans are organisms sharing characteristics with other organ-isms. The most important common feature of all organisms is life. This text recognizes six essential characteristics of life:
1. Organization refers to the specific interrelationshipsamong the parts of an organism and how those parts interact to perform specific functions. Living things are highly organized. All organisms are composed of one or more cells. Some cells, in turn, are composed of highly specialized organelles, which depend on the precise functions of large molecules. Disruption of this organized state can result in loss of function and death.
2. Metabolism (m̆e-tab′̄o-lizm) is the ability to use energyto perform vital functions, such as growth, movement, and reproduction. Plants capture energy from sunlight, and humans obtain energy from food.
3. Responsiveness is the ability of an organism to sensechanges in the environment and make the adjustments that help maintain its life. Responses include movement toward food or water and away from danger or poor environmental conditions. Organisms can also make adjustments that maintain their internal environment. For example, if body temperature increases in a hot environment, sweat glands produce sweat, which can lower body temperature down to the normal level.
1. Growth refers to an increase in size of all or part of theorganism. It can result from an increase in cell number, cell size, or the amount of substance surrounding cells. For example, bones become larger as the number of bone cells increases and they become surrounded by bone matrix.
2. Development includes the changes an organism undergoesthrough time; it begins with fertilization and ends at death. The greatest developmental changes occur before birth, but many changes continue after birth, and some continuethroughout life. Development usually involves growth, but it also involves differentiation. Differentiation is change in cell structure and function from generalized to specialized. For example, following fertilization, generalized cells specialize to become specific cell types, such as skin, bone, muscle, or nerve cells. These differentiated cells form tissues and organs.
3. Reproduction is the formation of new cells or neworganisms. Without reproduction of cells, growth and tissue repair are impossible. Without reproduction of the organism, the species becomes extinct.